This great plugin can be used to insert all kinds of things into posts, including ads. One nice side effect is that the WP editor will sometimes mangle complex html or inline javascript if you put it in the editor, but if you instead put it into one of the Ad boxes and let the plugin inject it for you, that issue goes away.

So simple yet so versatile, too! I used several other plugins that were much more complicated to use, or would only work with Adsense. This one not only will do more, but it is very intuitive and easy to use. I use it for all of my sites now!